1. Field of the Invention
This invention relates to dark current correction apparatus for eliminating a dark current signal component from an output signal of an image pick-up device having a plurality of pixels, which may for example be arranged in an array having, for instance, a matrix configuration.
2. Description of the Prior Art
Output signals (also referred to hereinafter as "imaging output signals") obtained from image pick-up devices (also referred to hereinafter as "imaging devices") such as video cameras are known to suffer from dark current arising from a number of causes, such as fluctuations in sensitivity of the imaging device or dark current effects, that is distortions in brightness over a wide extent of a viewing screen. For example, in a solid state imaging device, such as a charge coupled imaging device (CCD imaging device), it has been proposed to use a variety of image sensors, such as frame charge transfer, interline charge transfer or frame interline charge transfer type image sensors, according to which signal charges from pixels of the sensor, which are arranged in a matrix configuration, are transferred in the vertical direction and are sequentially read out by means of a horizontal transfer register, so that one horizontal line is read out during one horizontal scanning period, and the signal charges for the totality of the pixels of the device (and therefore for the totality of the pixels of a field or frame) are read out during one vertical scanning period, so as to produce an imaging output signal. Thus, a dark current proportional to the time during which the signal charges are transferred to the horizontal transfer register is added to the signal charges to give rise to sawtooth (serrated) changes in brightness during each vertical scanning period, that is dark current in the vertical direction. On the other hand, the dark current in the horizontal transfer register gives rise to sawtooth (serrated) changes in brightness during each horizontal scanning period, that is dark current in the horizontal direction.
In general, the above-mentioned dark current may be classified into white (modulation) according to which the output is lowered in peripheral regions of the viewing screen, and black (superposition) according to which the black level is not uniform throughout the viewing screen. Dark current correction processing is conventionally performed by mixing dark current correction signals into the imaging output signals in an analog fashion, using a multiplier for white and an adder for black. The dark current correction signals may be formed by generating sawtooth and parabolic signals for the horizontal and vertical directions, and combining these signals.
With the conventional dark current correction circuit, output levels of the sawtooth signal generators and the parabolic signal generators may be manually adjusted by means of a level control means, such as a volume knob. Thus, the output levels of the signal generators are adjusted manually to achieve optimum dark current correcting processing while reference is continuously made to a waveform monitor. U.S. Pat. No. 4,731,652, issued Mar. 15, 1988 to Yamanaka, discloses such an arrangement in which the levels of output signals of horizontal and vertical sawtooth and parabolic waveform generators are manually adjustable by means of gain control circuits comprising variable resistors.
If use is made of a three color CCD imaging system in which an object image is separated into color components, for example red, green and blue components, and images of the three color components are formed separately by three respective imaging devices, dark current correction has to be performed for each of the imaging devices.
Since, with the conventional dark current correction circuit, dark current correction is achieved by manually adjusting the output levels of the sawtooth and parabolic signal generators while continuously referring to a waveform monitor, the problem arises that a time-consuming adjustment operation by a skilled operator is needed for achieving accurate adjustment. This problem is serious even in the case of a single output imaging device. The problem is even more serious in the case of the three color CCD imaging system, in that the dark current correction that has to be performed for each of the three imaging devices is even more labor-intensive and time-consuming.